As the world's population grows and reserve supplies of clean water dwindle, the treatment of wastewater is becoming an increasingly important concern worldwide. Beginning in 1972 with the Clean Water Act, the United States substantially increased government regulations on wastewater treatment. These regulations along with aging water infrastructure, global population shifts, and the time pressures of existing treatment facilities further highlight the growing pressures on wastewater treatment industry.
Conventional wastewater treatment facilities employ a single or multi-stage process involving one or more of four main systems to remove the small amount of contamination contained in most wastewater. These systems include settling systems, biological systems, chemical treatment systems, and filtration/polishing systems.
Settling systems often comprise a primary treatment phase and involve the removal of solids suspended in wastewater. Such settling systems typically include settling ponds, primary classifiers, and the like. Larger solids are typically removed by screens, and smaller suspended solids by allowing them to settle to the bottom where they form sludge.
Biological treatment systems typically form a secondary phase of treatment, and are used primarily for the removal of organic materials. Typical biological treatment processes include facultative ponds, partially mixed lagoons, aerated lagoons, activated sludge, and the like. All of these systems employ microorganisms which remove harmful organics from the water. These microorganisms are contained in large ponds or lagoons where wastewater must sit for several days in order to allow sufficient time for the microorganisms to interact with and neutralize the contaminants contained in the water. Biological systems are highly effective and are used in approximately one third of all treatment systems worldwide. However, biological systems have certain limitations. For instance, if the microorganisms contained in the lagoons are shocked or killed by environmental changes or introduction of other agents, it can take weeks to produce a healthy colony of treatment bacteria. Furthermore, biological processes produce an enormous amount of toxic sludge (comprised largely of dead microorganisms and their waste products), which must be subsequently treated before disposal. Additionally, due to the large costs and time involved in constructing the facilities necessary for biological systems, they must be planned out well in advance of the anticipated need and may not respond to short term increases in the need for water treatment.
Various chemical treatment systems typically comprise a tertiary phase of wastewater treatment. These processes are designed to remove harmful bacteria from the wastewater, including any remnants of biological treatment processes. Additionally these processes are designed to remove dissolved contaminants contained in the wastewater stream. This phase can involve one or more of numerous processes including chlorination, ozonation, ion exchange or oxidation.
For most of the twentieth century, chlorination has been the most common method for removal of microorganisms from wastewater. While highly effective, chlorination has considerable drawbacks including the necessity of containing, transporting, and manipulating large quantities of potentially lethal chlorine. Frequently, the expense of maintaining appropriate chlorination facilities and training qualified personnel are cost prohibitive for smaller wastewater treatment facilities.
Another common chemical treatment method is the introduction of inorganic cationic coagulants such as aluminum salts and chlorides and sulfates of iron and calcium. Such treatments often create large amounts of residual sludge which must be disposed of. Further, chemical treatments are not well suited to influents of highly variable compositions.
The final phase of conventional wastewater treatment typically comprises polishing. During this phase the water is often filtered and treated depending on the particular waste-stream so that any unwanted coloration or odor is removed. Following this phase, the water is typically ready for discharge into the environment, consumption, or other use.